Ethylene is a major industrial commodity. A major source of it is steam cracking of saturated hydrocarbons. US 2011/0112314 describes a process for the production of ethylene in which an ethane containing feed is subjected to a cracking to produce ethylene and hydrogen. At least some of the hydrogen is reacted with carbon dioxide and/or carbon monoxide to form oxygenates which are then converted to olefin. It is also known to make ethylene by dehydration of ethanol. In addition, there are many other routes to olefins as illustrated in FIG. 3 of Tao Ren et al publication “Olefins from conventional and heavy feedstocks: . . . ” Energy 31 (2006) p 425-451. FIG. 3 clearly illustrates ethanol dehydration as a separate route to olefins as opposed to the steam cracking (SC) conventional route from a range of hydrocarbon feedstocks. Furthermore Ren describes in section 6.2 the latest developments in Naphtha cracking and in section 7.2 advanced olefins production via emerging catalytic olefin technologies wherein the combination of ethanol dehydration and steam cracking processes as an integrated scheme is not described nor is this mentioned as a potential means of reducing the overall energy footprint per tonne of olefin production.
Propylene is almost as significant an industrial commodity. It is obtained by a variety of routes including cracking. It is known also to prepare propylene by metathesis of other olefins such as ethylene and butene mixtures.
Steam cracking is a dominant route for producing olefin products, such as ethylene and propylene with these products being produced at very high purity such that they are suitable for use in polymerisation to polymers such as polyethylene and polypropylene. The purity requirements are extensive, an example would be the purity specification required for Aethylene Rohrlietungsgesellschaft (“the ARG company”), the operator of a common ethylene pipeline across parts of Europe supplying many ethylene polymerisation plants.
TABLE AKey specifications for ARG ethyleneComponentSpecificationEthylene99.9%volume minimumMethane & ethane1000ppm volume maxEthane500ppm volume maxAcetylene2ppm volume maxC3+10ppm volume maxHydrogen10ppm volume maxWater10ppm volume maxCarbon monoxide2ppm volume maxCarbon Dioxide5ppm volume maxOxygen5ppm volume max
TABLE BKey specifications for typical Polymer grade propyleneComponentSpecificationPropylene99.6%volume minimumTotal paraffin0.3%volume max
Residual other key components for polymer grade propylene are similar to those listed for ethylene with similar or lower specification levels.
Unless otherwise qualified proportions of gases as used herein are by volume as measured at STP.
A problem with known ethanol dehydration processes is that they are not totally selective for ethylene. As described in WO2010146332 one of the problems in preparing ethylene from ethanol is that C4 compounds, including C4 olefins, are also produced. For many applications it is necessary to remove these C4 compounds but it is expensive and complex to do so. In many cases the simplest and cheapest recovery schemes only provide a C4 stream suitable for use as a fuel for the process. Ethanol dehydration is a well known technology and much effort recently has focused on improvements to standalone dehydration process schemes for converting ethanol to ethylene, one such being the SINOPEC-ETO process described and illustrated as FIG. 3 by Teng in “New Olefin production technologies in SINOPEC SRIPT” Proceedings—World Petroleum Congress, 2008 19th( ):teng/1-teng/10 coden: wpcpau; issn: 0084-2176 wherein catalyst improvements are being sought to minimise the co-product yield and the fractions lost as Heavy Ends and Light Ends. The typical ethanol selectivity to ethylene and side products from a commercial catalyst is provided in prior art document “Ethylene from Ethanol” process brochure from Chematur Engineering Group on web address; www.weatherlyinc.com/sok/download/Ethylene_rev_0904.pdf